An optical signal propagated through a channel is subjected to a waveform distortion due to the effects of different wavelength dispersion depending on the kind of channel fiber, the transmission distance, the signal waveform and the like. For this reason, a general optical transmission apparatus compensates for the waveform distortion by using a Dispersion Compensation Fiber (DCF) having a characteristic complementary to that of the channel.
In order to achieve the complementary characteristic described above, the DCF varies a refractive index ratio of glass by an additive material, and fabrication parameters and a refractive index profile of the DCF are complex. Because of the complex structure of the DCF, an optical loss in the DCF increases with aging.
FIG. 1 is a diagram illustrating a structure of an example of a conventional optical amplifier module. An optical amplifier module 1 includes therein two optical amplifiers 2 and 3. A DCF module 4 is connected between the two optical amplifiers 2 and 3, in order to compensate for the wavelength dispersion generated in the channel.
On the other hand, a Japanese Laid-Open Patent Publication No. 2000-307552, for example, proposes maintaining an optical output level of wavelength division multiplexed signal light in a wavelength band constant, by inputting pseudo signal lights respectively having wavelengths shorter than and longer than the wavelength band of the wavelength division multiplexed signal light to an optical amplifier, and branching an output of the optical amplifier, in order to control the optical amplifier by the short wavelength and the long wavelength.
The optical amplifier module 1 amplifies the signal light that has been subjected to the loss in the channel by the optical amplifier 2 located at a stage preceding the DCF module 4, and amplifies the signal light that has been reduced in the channel and the DCF module 4 by the optical amplifier 3 located at a stage subsequent to the DCF module 4. The optical amplifiers 2 and 3 respectively at the stages preceding and subsequent to the DCF module 4 have input dynamic ranges on the order of several dB. For this reason, even if the optical loss of the DCF module 4 changes, the optical amplifiers 2 and 3 operate in a normal manner as long as the input dynamic ranges thereof are satisfied.
In addition, because the input and output levels of the optical amplifier module 1 change depending on a change in the number of wavelengths, the loss of the DCF module 4 itself is usually not measured. Consequently, no alarm will be generated even if the loss in the DCF module 4 increases, and in a worst case scenario, there was a problem in that the increase in the loss in the DCF module 4 will not be detected until a signal disconnection occurs.